Not uncommon phenomenon. It depends on the powder used, actually. In most any cartridge, one can find a very slow powder that will work very well if packed in and the bullet beat into position. A 'fast' powder for the same cartridge will require less volume (or weight, depending on how one measures) and leave extra space.

I tend to use slower powders and load them full. I think it gives me a more uniform burn rate, from round to round.

Hatcher's Notebook mentions this. Some early loads for the 03 Springfield would give different velocities if the rifle were held muzzle up vs. being pointed at the ground prior to firing. Seems to be the position of the powder in the case affects the burn results.

depends on the powder used. ideally you want the case full enough for a slightly compressed load. most rifle cartridges do better with powders that completely fill the case.

the reason you get some shake with surplus and el-cheapo type ammo is they use super cheap ball powder that tends to burn faster with minimal charges to save costs.

one of the dangers of minimal charges is detonation. this happens when there is too much air space in the case and instead of going down the barrel the powder charge builds up too much pressure to fast and it overloads the case causing a kaboom. however that wont happen in factory ammo or with small cases like 308 30-06 or 7.62x54r. its a danger in big belted magnum cartridges like 458 winmag 458 lott 50bmg 338 lapua etc

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"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith

The problem with being stupid is you cannot simply decide to stop doing dumb things...